The price you pay is your time. Donation-based products are financed in this way. You spend your time, and those who want you to spend your time (there may be another reason, that's subjective) spend their money for that. And you can donate too. Volunteering is one of the basic principles of the free market. So that slogan doesn't work for open source software.
Discussion
Posthog is open source, I use the open-source version of that, but I'm pretty sure I'm the product there one way or another. Same with Supabase.
For me, if it's a closed loop between passionate devs and donations, with no other money in the system, then I agree with you. And it doesn't have to be open source, just that closed loop.
But here in nostr is not such as closed loop. There is a lot of money here, clearly. All us free users, we're all the product of something, just not clear what yet.